This invention relates to a device which is adapted to receive electromagnetic radiation. The invention is particularly, but not exclusively, concerned with systems for receiving microwave radiation.
The term microwave is generally understood to refer to the part of the electromagnetic spectrum between infra-red radiation and radiowaves. Typically this is stated to be substantially in the frequency range 1 to 300 GHz, although sometimes it is stated to be in the frequency range 0.2 to 300 GHz. It includes that part of the spectrum referred to as millimeter wave (having a frequency in the range 30 to 300 GHz).
Communications systems have been proposed in which one or more communications channels are transmitted in a particular direction in the form of a modulated electromagnetic beam propagating through free space, for example the atmosphere. An advantage of such a directional communications system over a communications system which broadcasts omnidirectionally is that there is a greater degree of security in that the communications channel or channels can be directed towards a particular location. For example, if omnidirectional transmission is used, not only can others receive the transmission readily bat the presence, and possibly the location, of the transmitting station can be determined.
In one embodiment of a communications system, units which are spatially separated need to communicate with each other. If any of the units are mobile, then the directional communications channels could come from any direction in an azimuthal plane. It then becomes important to establish the direction from which a communications channel is coming in order that a reply can be sent in the correct direction. Although this can be done by having a number of antennas pointing in different directions, a single omnidirectional antenna is preferred.
In a particular embodiment of a communications system a typical interrogation sequence might proceed as follows. The station to be interrogated is identified and an interrogating station transmits an interrogation signal. The interrogation signal typically has a first portion simply comprising a pulse of electromagnetic radiation which can be detected by the station being interrogated to know that an interrogation sequence has begun. It is not necessary for the pulse to contain any data. It may be about 100 xcexcs long. Following the first portion, a second portion containing data is transmitted, for example in a burst 300 to 400 xcexcs long. Therefore, the station being interrogated has 400 to 500 xcexcs to find out the direction from which the interrogation signal is originating in order that it can send its response signal in the correct direction.
Generally, there are systems other than communications systems in which determining the direction of origin of radiation is desirable. Such systems may be tracking systems.
According to a first aspect of the invention there is provided a system for receiving radiation comprising polarisation varying means to induce an angularly dependent variation in polarisation state and detection means to measure the polarisation state of the radiation to determine the direction from which it originates.
Preferably the polarisation varying means produces a relative change between at least two components of the radiation.
In one embodiment the polarisation varying means produces a differential phase delay between the two components. This may have the effect of converting linearly polarised radiation into circularly polarised radiation and/or vice versa. In another embodiment the polarisation varying means produces a change in the relative magnitudes of the two components. This may have the effect of rotating the polarisation state of the radiation. This may particularly have the effect of rotating the polarisation of linearly polarised radiation. Both effects may be induced by the polarisation means.
Preferably the polarisation varying means comprises a phase plate which induces a variable phase delay dependent on the angular position the radiation passes through the plate. Conveniently the phase delay is induced by the phase plate having a physical thickness which varies angularly about the phase plate. Preferably the two components pass through the phase plate. Alternatively only one component passes through the phase plate with the other serving as a reference.
Preferably the phase plate has birefringent properties which cause the components to propagate differently through the plate. The components may propagate through the plate at different speeds.
Preferably the system comprises a splitter which splits the radiation into the two components. The split may occur before the radiation has had induced in it a variation in polarisation state, or after.
Preferably the system comprises a radiation collector. This may be a horn. Preferably the radiation is microwave radiation. Most preferably it is millimetric radiation. Most preferably of all it is radiation in the Ka band (26.5 to 40 GHz). Preferably the system is a receiver in a communications system. It may be part of a composite transmitter and receiver unit.
According to a second aspect of the invention there is provided a communications system comprising a radiation receiving system in accordance with the first aspect of the invention.
According to a third aspect of the invention there is provided a method of determining the direction from which radiation originates comprising the steps of inducing a variation in polarisation state of the radiation the variation being angularly dependent and measuring the polarisation state.